Microwave heating of frozen food products is characterized by a slow and largely non-uniform reheating first step before the product is thawed, followed by a more rapid second step wherein the thawed parts of the product are heated. In fact, the non-uniformity of the heating before the thawing is translated and even amplified during the second heating step. Therefore, there is a need to focus on the frozen state and to modify the physical properties of the frozen product in order to increase the rate of thawing and to reduce the temperature gradients between the various parts of the food product during the first microwave heating step.
It has been found that frozen foods have very low dielectric parameters at the microwave heating frequency of 2.45 GHz, ranging from 1.1 to 2.5 for the real part of the permittivity, .epsilon.', and from 0.01 to 0.0001 for the imaginary part, .epsilon." (microwave absorption). These values are both close to those of gases and air at ambient conditions. The microwave heating rate that is induced by dipolar absorption is proportional to the microwave absorption .epsilon.", and therefore is very small for all frozen products. In addition, because of the low values for the dielectric parameters, the impedance of the whole frozen product does not affect the microwave field distribution in the oven cavity, resulting in a heating pattern that is largely non-uniform.
U.S. Pat. No. 4,428,971 to Havette et al. discloses a process for the preparation of a composition for frozen or deep-frozen souffles. The patent relates to the incorporation of carbon dioxide in the proportion of 0.5 to 2% by weight of the souffle in order to promote better rising during baking in a conventional oven and to reduce the likelihood of the souffle's collapsing when served. According to Havette, a small amount of carbon dioxide is fully dissolved into the souffle base which is then stored in a freezer. The dissolution of the carbon dioxide is achieved by injecting carbon dioxide gas into the product or by mixing solid carbon dioxide with the souffle recipe until complete dissolution of the gas is achieved. In both cases, the product obtained after this processing step is a "batter" with some gas dissolved in it. Macroscopically, this carbon dioxide gas does not form gas pockets in the "batter" because of the small quantity of solid carbon dioxide used and because of the way it is incorporated into the composition. In Havette, the carbon dioxide is mixed in small amounts at low speed until the gas has completely dissolved. When the frozen composition is reheated by baking, the dissolved gas expands so that the whole upper part of the souffle rises and does not collapse when it is served. The method of cooking a souffle disclosed by Havette is adapted to conventional ovens, however, and would not be suited to microwave heating. The process of Havette does not utilize a structure which is microscopically modified so as to provide significant improvements for microwave heating.
U.S. Pat. No. 5,624,700 to Ogden discloses the incorporation of relatively large amounts of carbon dioxide in solid or semi-solid foods during the final processing steps for making a semi-solid or solid spoonable food. The food products as manufactured, however, are carbonated cream or ice cream products which are not intended to be processed in microwave ovens.
GB Patent No. 1,005,399 to American Machine and Foundry Company discloses a method of enhancing the flavor of a dairy dessert by introducing carbon dioxide into the dessert to lower the initial pH value of the mixture and provide a dessert having greater zest through carbonation.
JP 59146542 A1 to Sawaguchi Kazuore discloses a mixture for making carbonated ice cream, sherbet, or frozen yogurt.
JP 6062754 A to Kanebo Ltd relates to the production of carbonated ice candy.
Despite these disclosures, there remains a need for improved microwave reheatable frozen products, and the present invention satisfies this need.